Chooks with red backsides

First time poster but long time reader. I live in Australia and have 6 backyard chooks, 4 of them are 1 1/2 years old and 2 newbies are just over 20 weeks. The 4 older girls have been developing red raw areas over their backsides and legs that are devoid of feathers and look quite red. My chicken supplier thought that it may indicate worms but I have treated with Piperazine to no effect. I have attached some photos and am hoping that you may be able to help if you have seen something similar.

You will want to follow up the piperazine with another wormer to get the rest of the types of worms. Damage from mites and lice commonly first shows up on rear ends. You will want to check or treat for leg mites as well.